Quantum non-demolition measurement of a superconducting two-level system Delft|NTT By minimizing disturbance to the system under investigation, quantum nondemolition measurement (QND) can provide particularly clear signatures of quantum coherence. In Nature Physics and cond-mat 0611505, Lupascuet al. demonstrate nondemolition measurement of superconducting qubits coupled to a nonlinear resonator. "The high correlation between measurement results demonstrates the quantum nondemolition nature of the readout method. The fact that quantum nondemolition measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum information processing protocols such as state preparation and error correction. "cf. also Kavli Institute announcement in TU Delta (in Dutch).

20070122

High-speed linear optics quantum computing using active feed-forward measurement Vienna In Nature445, 65-69 and concurrent press summary, Zeilinger's group reports experimental demonstration of feedforward error correction via one-way, highly-entangled cluster states in linear optics. "With present technology, the individual computational step can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states."

20070119

Proton Tunneling in Molecular Biophysics Rensselaer RPI researchers have employed the SCOREC supercomputing cluster to conduct advanced modeling of protein folding dynamics which incorporates quantum mechanical effects to study the influence of proton tunneling in enzyme catalysis. The group's initial study of intein's role in C-termini protein folding will be used to develop nanoscale switches for applications ranging from drug delivery to novel sensors.

20070118

Measurement-based Quantum Computing with Superconducting Charge Qubits RIKEN Wang, You and Nori report on measurement-based preparation of superconducting cluster states. "The measurement of the current of a few parallel Josephson-junction qubits realizes a novel type of quantum-state selector. Using this selector, one can produce various quantum entangled states and also realize a controlled-NOT gate without requiring an exact control of the interqubit interactions. In particular, cluster states for quantum computation could be produced with only single-qubit measurements."

Measuring the Size of a Schrödinger Cat State München "We propose a measure for the "size" of a Schrödinger cat state, i.e. a quantum superposition of two many-body states with macroscopically distinct properties, by counting how many single-particle operations are needed to map one state onto the other. This definition gives sensible results for simple, analytically tractable cases and is consistent with a previous definition restricted to Greenberger-Horne-Zeilinger-like states. We apply our measure to the experimentally relevant, nontrivial example of a superconducting three-junction flux qubit put into a superposition of left- and right-circulating supercurrent states and find this Schroedinger cat to be surprisingly small."